Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Initially, electrodeposition was conducted with the workpiece to be coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972 cationic electrodeposition was introduced commercially and has continued to gain in popularity. Today, cationic electrodeposition is by far the prevalent method of electrodeposition. For example, a cationic primer coating is applied by electrodeposition to more that 80 percent of all motor vehicles produced throughout the world.
Many industrial applications utilize electrodepositable acrylic coatings, crosslinked with aliphatic isocyanates. Acrylic coatings are typically more stable to ultraviolet radiation-induced degradation than their epoxy-based counterparts. Such coatings are frequently used in one-coat applications, or in conjunction with a clear topcoat. Electrodepositable primer coating compositions, particularly those used in the automotive industry, typically are corrosion-resistant epoxy-based compositions crosslinked with aromatic isocyanates. If exposed to ultraviolet energy, such as sunlight, these compositions can undergo photodegradation. In most applications, a primer-surfacer is spray-applied directly to the cured electrodeposited coating prior to application of one or more topcoats. The primer-surfacer can provide a variety of properties to the coating system, including protection of the electrodeposited coating from photodegradation. Alternatively, one or more topcoats can be applied directly to the cured electrodeposited coating and in such instances, these topcoats typically are formulated such that the topcoat(s) provide sufficient protection to prevent photodegradation of the electrodeposited primer coating. If the topcoat(s) do not provide sufficient protection, photodegradation of the electrodeposited primer coating can result in delamination of the subsequently applied topcoats from the cured electrodeposited primer coatings producing catastrophic failure of the cured coating system.
The presence of metal salts, in particular iron salts, in an electrodeposition bath is known to have a detrimental effect on the durability of an electrodeposited coating. Iron ions are commonly found in electrodeposition baths that are in contact with ferrous materials. Common sources of iron contamination include fallen metal substrate in the tank, weld splatter, metal flakes and dust from grinding of the metal part prior to electrodeposition, pretreatment chemical carryover, pump housings, and pipes, among other sources.
Attempts to reduce or eliminate metal ions, particularly iron, in an electrodeposition bath have been reported in the art. One method involves addition of an iron chelator to an affected electrodeposition bath. The chelator is chosen such that it forms water-soluble complexes with iron ions or heavy metal ions in general, which are then physically removed from the bath by ultrafiltration. One difficulty with this approach is that many chelators that form soluble complexes are also agressive toward dissolution of the substrate being coated, thus requiring very strict control of the level of chelator present as well as timely removal of resultant complexes from the electrodeposition bath.
Also known in the art is an electrodepositable coating composition comprising an iron ionization-inhibitor, which may assist in the maintenance of an acceptable level of iron by reducing the rate of dissolution of metallic iron. Compounds can be included in an electrodepositable bath composition which either inhibit dissolution or passivate the surface of the metallic substrate.
The aforementioned prior art addresses methods to remove metal ions from, or reduce the amount of metal ions introduced into an electrodeposition paint bath. The described methods require careful monitoring and control of additives intended to remove the metal ions. There remains a need in the coatings industry for a method to control the deleterious effect of metal ions with minimal maintenance.